1. Field of the Invention
The present invention relates to a catalyst system for vehicle exhaust gas purification devices, exhaust gas purification device using the same and method for purification of exhaust gases, more specifically a catalyst system for vehicle exhaust gas purification devices which exhibit excellent capacity of removing nitrogen oxide emissions from vehicle exhaust gases when they are brought into contact with the catalyst even when hydrocarbon concentration in exhaust gases varies, exhaust gas purification device using the same and method for purification of exhaust gases.
2. Description of the Prior Art
Exhaust gases discharged from combustion devices, e.g., vehicle internal combustion engines or the like and boilers, contain hazardous substances, e.g., hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), and various techniques have been proposed for purification of these hazardous substances. As one of these techniques, one which has been studied disposes a catalyst in an exhaust gas passage to purify hazardous components of exhaust gases.
In particular, such a catalytic technique, when applied to purification of exhaust gases discharged from vehicles, is required to efficiently purify these hazardous gases (CO, HC and NOx) while flexibly coping with rapidly changing concentrations of these gases, ranging from several ppm to several %.
In the catalytic exhaust gas purification techniques, oxygen present in exhaust gases plays important roles for promotion of CO and HC oxidation and NOx reduction. It is however difficult for a vehicle to keep constant engine combustion conditions, because of changing driving conditions resulting from changing traffic conditions, with the result that oxygen concentration changes every second. Therefore, purification of hazardous gases by using oxygen will have a deteriorated purification efficiency, when oxygen concentration drops.
In order to cope with these situations, the catalyst is incorporated with an oxygen storage/release component (hereinafter sometimes referred to as OSC), which releases oxygen which it stores into exhaust gases containing oxygen at an insufficient concentration, to improve hazardous gas purification capacity.
CeO2 powder has been used for an OSC, because of its high oxygen storage/release capacity, and demonstrating an effect of improving exhaust gas purification efficiency. Extensive studies have been made on improvement of oxygen storage capacity and release characteristics of CeO2-base powders, e.g., CeO2/ZrO2-base powder, and also on exhaust gas purification catalysts which are incorporated with an OSC as a promoter.
It is discussed that a cerium/zirconium-base compound oxide of specific atomic composition is effective as an oxygen storage/release component for adjusting exhaust gas purification catalyst functions of oxidation of HC and CO and reduction of NOx.
An exhaust gas purification catalyst mounted in a vehicle is disposed under a floor, at which temperature is relatively low, or immediately below the engine where it is exposed to higher temperature of exhaust gases immediately after being discharged from the engine. Exhaust gases, which are at high temperature immediately after being discharged from an engine, lose temperature when they reach the underfloor area. These conditions are not favorable for exhaust gas purification, because many exhaust gas purification catalysts generally exhibit a high activity at above a certain temperature. Nevertheless, however, a catalyst disposed under a vehicle floor is required to exhibit purification capacity even under decreased temperature conditions.
On the other hand, the one disposed immediately under an engine may have the exhaust gas purification catalyst components sintered when exposed to exhaust gases which may be at above 1000° C. Therefore, such a catalyst is required to have a stable exhaust gas purification capacity even under severe high-temperature conditions by suppressing sintering of the components.
One type of these catalysts simultaneously performs oxidation of CO and HC and reduction of NOx for purification of exhaust gases. Such a catalyst is known as a three-way catalyst (TWC) (a device in which a TWC is mounted is hereinafter sometimes referred to as a TWC device). The TWCs generally have a structure with a structural carrier of cordierite, which is coated with a slurry comprising a base material for a porous carrier (e.g., of alumina) impregnated with a noble metal, e.g., platinum, rhodium, palladium or the like.
Such a TWC removes a reducing component such as CO, HC or the like by oxidation with oxygen in exhaust gases and NOx by reduction. As discussed above, however, oxygen concentration in exhaust gases changes every second, and catalysts exhibiting a purification capacity under these conditions are in demand. Therefore, an exhaust gas purification catalyst is incorporated with cerium oxide, cerium/zirconium-base compound oxide (Patent Document 1) as an oxygen storage/release material to improve its hazardous component purification capacity by releasing oxygen which it stores into exhaust gases to increase oxygen content of exhaust gases containing oxygen at an insufficient concentration. Incorporation of an oxygen storage/release material is mainly intended to reduce extent of oxygen concentration changes.
However, the markets are demanding OSC materials capable of release oxygen in a lower temperature range, having a higher oxygen storage/release capacity and securing a longer service life of the catalysts in which they are incorporated to cope with the exhaust gas regulations, which are becoming more and more stringent. At the same time, exhaust gas purification catalysts are required to be more resistant to temperature. Under these situations, the inventors of the present invention have developed an OSC material exhibiting an excellent performance even at a high temperature, and proposed a cerium/zirconium-base compound oxide which is not sintered at a high temperature of 1000° C. or higher (Patent Document 2).
In some cases, catalysts exhibiting different functions in accordance with exhaust gas components to be purified are disposed at different places in a vehicle exhaust gas passage to constitute a vehicle exhaust gas purification device.
Two or more types of TWC themselves are frequently disposed in an exhaust gas passage also in a gasoline engine. For example, Patent Document 3 discloses a device which purifies exhaust gases in two stages, with a more heat-resistant catalyst in the first stage where it is exposed to exhaust gases of higher temperature, and a common TWC in the lower-temperature second stage to completely oxidize HC, which is partly oxidized in the first stage. Patent Document 4 discloses a device working also in two stages with a TWC in the first stage and composite catalyst of HC adsorbent and TWC in the second stage, in an attempt to greatly improve rate of purification of HC in exhaust gases.
When a catalyst is disposed in an exhaust gas passage of limited space and shape, similar catalysts are disposed in two or more places to provide an active area sufficient for exhaust gas purification. For example, Patent Document 4 discloses a TWC device with two or more catalysts to purify exhaust gases.
It is accepted that a conventional TWC can adequately purify three components of HC, CO and NOx only in a narrow theoretical air/fuel ratio range around 14.6 (the range is referred to as a window). However, a vehicle must work under varying conditions resulting from varying driving conditions, discussed earlier, and under lean conditions to improve mileage, in order to cope with heightened interests in environmental problems in these days, where the lean condition may result from a fuel cut condition, under which supply of fuel to a combustion chamber is temporarily suspended, and also from a lean-burning condition. At a high theoretical air/fuel ratio, NOx is produced more and difficult for a conventional TWC to purify.
Vehicles of improved fuel economy have been demanded also viewed from depletion trends of oil resources and global warming, and lean-combustion engines have been attracting more attention also for gasoline engines. In lean combustion, an exhaust gas atmosphere created by a vehicle being driven is slanted to an oxygen-rich state (sometimes referred to as lean atmosphere) from that associated with combustion at the theoretical air/fuel ratio (sometimes referred to as stoichiometric condition). A conventional TWC, when it works in a lean atmosphere, involves a problem of insufficient NOx purification, because NOx is produced more in such an atmosphere of higher oxygen concentration. Therefore, catalysts which can purify NOx even in a lean atmosphere have been studied (refer, e.g., to Patent Document 5).
More recently, however, vehicles have been demanded to have still improved fuel economy, and driven under lean atmospheres and fuel-cut conditions more frequently. At the same time, upsurge of environmental awareness has been requiring still deeper purification of NOx, and further improvements of NOx purification capacity of TWCs, in particular    Patent Document 1: JP-B 6-75675    Patent Document 2: WO2006/030763    Patent Document 3: JP-A 11-123331    Patent Document 4: JP-A 7-144119    Patent Document 5: JP-A-10-192713